U.S. patent application number 09/502919 was filed with the patent office on 2002-01-10 for use of rare earth metal salt solutions for sealing of anodized aluminum for corrosion protection and paint adhesion.
Invention is credited to Dull, Dennis L., Mansfeld, Florian B..
Application Number | 20020003093 09/502919 |
Document ID | / |
Family ID | 26723690 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020003093 |
Kind Code |
A1 |
Dull, Dennis L. ; et
al. |
January 10, 2002 |
Use of rare earth metal salt solutions for sealing of anodized
aluminum for corrosion protection and paint adhesion
Abstract
A process for sealing the surface coating formed by anodizing an
aluminum or aluminum alloy substrate (for example, aerospace,
commercial, and architectural products), the process including the
steps of: (a) providing an aluminum or aluminum alloy substrate
with a surface coating formed thereon by anodizing the aluminum or
aluminum alloy substrate; (b) providing a sealing solution
comprising a dilute solution of a rare earth metal salt selected
from the group consisting of cerium salts and yttrium salts; and
(c) contacting the substrate with the sealing solution for a
sufficient amount of time to seal the surface coating on the
substrate. Also disclosed is a chemical sealing solution for
sealing the surface coating formed by anodizing an aluminum or
aluminum alloy substrate, the solution being a dilute solution of a
rare earth metal salt selected from the group consisting of cerium
salts and yttrium salts.
Inventors: |
Dull, Dennis L.; (Sumner,
WA) ; Mansfeld, Florian B.; (Santa Monica,
CA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Family ID: |
26723690 |
Appl. No.: |
09/502919 |
Filed: |
February 11, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09502919 |
Feb 11, 2000 |
|
|
|
09076310 |
May 11, 1998 |
|
|
|
6248184 |
|
|
|
|
60046234 |
May 12, 1997 |
|
|
|
Current U.S.
Class: |
205/203 ;
205/204 |
Current CPC
Class: |
C25D 11/246 20130101;
C23C 28/042 20130101; C23C 30/00 20130101 |
Class at
Publication: |
205/203 ;
205/204 |
International
Class: |
C25D 011/18 |
Claims
What is claimed is:
1. A process for sealing the surface coating formed by anodizing an
aluminum or aluminum alloy substrate, said process comprising the
steps of: (a) providing an aluminum or aluminum alloy substrate
with a surface coating formed thereon by anodizing said aluminum or
aluminum alloy substrate; (b) providing a sealing solution
comprising a dilute solution of a rare earth metal salt selected
from the group consisting of cerium salts and yttrium salts; and
(c) contacting said surface coating on said substrate with said
sealing solution for a sufficient amount of time to seal said
surface coating on said substrate.
2. The process of claim 1 wherein said sealing solution comprises a
dilute solution of cerium salts.
3. The process of claim 1 wherein said sealing solution comprises a
dilute solution of yttrium salts.
4. The process of claim 1 wherein said sealing solution comprises a
dilute solution of a rare earth metal salt selected from the group
consisting of cerium nitrate, yttrium sulfate, and cerium
sulfate.
5. The process of claim 1 wherein the chemical concentration of the
dissolved rare earth metal salt in the sealing solution is from
about 10 mM to about 350 mM.
6. The process of claim 1 wherein the pH of said sealing solution
is from about 3.0 to about 9.0.
7. The process of claim 1 wherein the immersion time in said
sealing solution is from about 10 minutes to about 60 minutes.
8. The process of claim 1 wherein the temperature of the sealing
solution is from about 60.degree. C. to the boiling temperature of
the sealing solution.
9. A chemical sealing solution for sealing the surface coating
formed by anodizing an aluminum or aluminum alloy substrate, said
solution comprising a dilute solution of a rare earth metal salt
selected from the group consisting of cerium salts and yttrium
salts.
10. The chemical sealing solution of claim 9 wherein said sealing
solution comprises a dilute solution of cerium salts.
11. The chemical sealing solution of claim 9 wherein said sealing
solution comprises a dilute solution of yttrium salts.
12. The chemical sealing solution of claim 9 wherein said sealing
solution comprises a dilute solution of a rare earth metal salt
selected from the group consisting of cerium nitrate, yttrium
sulfate, and cerium sulfate.
13. The chemical sealing solution of claim 9 wherein the chemical
concentration of the dissolved rare earth metal salt in said
sealing solution is from about 10 mM to about 350 mM.
14. The chemical sealing solution of claim 9 wherein the pH of said
sealing solution is from about 3.0 to about 9.0.
15. The chemical sealing solution of claim 9 wherein the
temperature of said sealing solution is from about 60.degree. C. to
the boiling temperature of the sealing solution.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
application No. 60/046,234 filed on May 12, 1997.
BACKGROUND OF THE INVENTION
[0002] This environmental-quality invention is in the field of
sealing the surface coatings produced by anodizing aluminum and
aluminum alloy substrates (for example, aerospace, commercial, and
architectural products). The invention produces sealed anodization
coatings exhibiting good corrosion resistance performance while
maintaining acceptable levels of paint adhesion performance.
[0003] The International Agency for Research on Cancer has
identified both chromium and nickel compounds along with many other
pollutants as confirmed human carcinogens. The Boeing Company
(Boeing), along with many other companies, has voluntarily agreed
with the U.S. Environmental Protection Agency (EPA) to reduce the
use of the seventeen most hazardous pollutants which include these
compounds. Currently, the only approved sealing solution for the
coating produced by the boric acid-sulfuric acid anodizing process
is a dilute (45-75 ppm) chromate seal solution. The purpose of the
chromate sealing solution is to hydrate surface oxide while
entrapping the hexavalent chromium. The hexavalent chromium acts as
a corrosion inhibitor to further enhance the corrosion resistance
of the anodized coating. Using this dilute chromate seal solution,
production operations can use the boric acid-sulfuric acid
anodizing process on aluminum alloys 2024, 6061, and 7075 and
produce parts that pass a two-week salt spray test and meet the
requirements for paint adhesion. Unfortunately, the dilute chromate
sealing solution is a hazardous pollutant.
[0004] The unsealed aluminum oxide produced by anodizing is usually
modeled as two oxide layers on an aluminum substrate. The inner
layer is a thin continuous barrier layer of less than 500 angstroms
thickness. The outer layer is a discontinuous coating with pores
that may penetrate from the outside surface to the barrier layer.
These pores are the source of potential corrosion pitting problems
that occur in salt spray and other atmospheric environments. In the
dilute chromate seal solution process, these aluminum oxide pores
are hydrated with entrapped hexavalent chromium. This filling of
the pores enhances the corrosion protection of the anodized coating
on the aluminum substrate.
[0005] In B. Yaffe, Metal Finishing, May 1990, vol. 41 (1990), the
author reviews the known methods of sealing anodized aluminum, such
as sealing in steam and hot water, nickel acetate, dichromate, and
various cold sealing methods. Some of the newer sealing methods
have been developed due to environmental concerns and the desire to
lower costs. Cold sealing in nickel fluoride has been introduced to
lower these costs. However, health hazards have been observed
recently for nickel salts, which can cause allergic contact
dermatitis. In NASA Tech Briefs, May 1995, a sulfuric acid
anodizing process with a lower temperature nickel acetate seal is
described. This process produces thin anodized layers that are not
detrimental to the fatigue properties of the aluminum substrate,
but does not address the health hazards due to the use of nickel
salts. In Boeing's boric acid-sulfuric acid anodizing process,
anodized layers of about 1 .mu.m thickness are produced, which are
then sealed using a dilute chromate solution (as described in
Boeing Process Specification BAC 5632, "Boric Acid-Sulfuric Acid
Anodizing").
[0006] In a study to develop an overall corrosion protection system
for aluminum alloys, co-inventor Mansfeld developed a treatment for
commercial aluminum alloys using two rare earth metal salt
solutions that produced surfaces with excellent resistance to
pitting (see Mansfeld et al. U.S. Pat. No. 5,194,138, "Method For
Creating A Corrosion-Resistant Aluminum Surface"). For commercial
aluminum alloys having a high copper content, co-inventor Mansfeld
developed an additional pre-treatment to remove copper from the
outer surface to further enhance corrosion protection (see Mansfeld
et al. U.S. Pat. No. 5,582,654, "Method For Creating A
Corrosion-Resistant Surface On Aluminum Alloys Having A High Copper
Content").
BRIEF SUMMARY OF THE INVENTION
[0007] In one aspect, the invention is a process for sealing the
surface coating formed by anodizing an aluminum or aluminum alloy
substrate (for example, aerospace, commercial, and architectural
products), the process including the steps of:
[0008] (a) providing an aluminum or aluminum alloy substrate with a
surface coating formed thereon by anodizing the aluminum or
aluminum alloy substrate;
[0009] (b) providing a sealing solution comprising a dilute
solution of a rare earth metal salt selected from the group
consisting of cerium salts and yttrium salts; and
[0010] (c) contacting the substrate with the sealing solution for a
sufficient amount of time to seal the surface coating on the
substrate.
[0011] In another aspect, the invention is a chemical sealing
solution for sealing the surface coating formed by anodizing an
aluminum or aluminum alloy substrate, the solution being a dilute
solution of a rare earth metal salt selected from the group
consisting of cerium salts and yttrium salts.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The rare earth metal salt sealing solutions described herein
provide an alternative to the commonly-used chromate-type seal
solutions for the boric-sulfuric acid anodizing process, for the
sulfuric acid anodizing process, and for the chromic acid anodizing
process. These rare earth metal salt sealing solutions contain low
toxicity materials that may be disposed of easily.
[0013] Aluminum alloys anodized by the boric acid-sulfuric acid
anodizing process and then sealed with a rare earth metal salt
sealing solution meet the same performance requirements called out
for these alloys when sealed using a dilute chromate seal solution.
These tests include the salt-spray test conducted in accordance
with ASTM B117 ("Standard Test Method of Salt Spray (Fog) Testing")
and the paint adhesion test conducted in accordance with Boeing
Support Standard BSS 7225 ("Adhesion, Tape Test"). Test panels of
aluminum alloys 6061 and 7075 passed the 336-hour salt spray test
with less than one pit per 10 sq. in., which is the passing
criterion. Test panels of aluminum alloy 2024 require further
optimization since they had about two pits per 10 sq. in. Test
panels of anodized aluminum alloys 2024, 6061, and 7075 sealed with
rare earth metal salt sealing solutions and then sprayed with a
paint qualified under Boeing Material Standard BMS 10-11 ("Chemical
and Solvent Resistant Finish") passed the dry adhesion, 24-hour wet
adhesion, and seven-day adhesion tests. There was no primer lift
off from any panel in any of the three adhesion tests although up
to {fraction (1/32)} in. primer lift off beyond the scribe is
acceptable.
[0014] The objective of this invention is to replace the current
dilute chromate sealing solution with an equivalent-performing or
better non-chromate seal solution using either a similar or an
alternative inhibitive approach and chemical substances that are
not currently or foreseen to be listed as toxic by the EPA. Also,
our objective is to minimize upset to the current boric
acid-sulfuric acid anodizing process by providing a seal whereby
parts need not be sorted due to alloy composition.
[0015] We conducted research to evaluate rare earth metal salt
sealing solutions such as cerium salts, yttrium salts, and others
as a replacement to the currently successful dilute chromate seal
solution used for sealing the coatings produced by the boric
acid-sulfuric acid anodizing process. More specifically, we
included: yttrium acetate, yttrium sulfate, yttrium chloride,
cerium nitrate, cerium acetate, cerium sulfate, nickel fluoride (a
European standard), boiling water, and dilute chromate seal
solution as our standard. The aluminum alloys included 2024, 6061
and 7075. The test methods included electrochemical impedance
spectroscopy (EIS) and optical microscopy examination at 30.times.
of the panels after immersion in 0.5N NaCl solutions.
Sealing Process
[0016] The sealing process for an anodized aluminum alloy part is
as follows:
[0017] Sealing:
[0018] Immerse parts in the sealing solution at the specified
temperature for the prescribed period of time.
EXAMPLE 1
[0019] Panels (4 in..times.6 in.) of aluminum alloys 2024, 6061,
7075 were coated in accordance with the boric acid-sulfuric acid
anodizing process as described in Boeing Process Specification BAC
5632, "Boric Acid-Sulfuric Acid Anodizing". Then a 50 mM cerium
nitrate sealing solution (mM is the abbreviation for millimolar)
was prepared by dissolving the cerium nitrate salt in distilled
water and adjusting to pH 6 using nitric acid at room temperature.
The solution was heated to the boiling temperature which is
approximately 100.degree. C. Panels were immersed in the sealing
solution for 30 minutes.
EXAMPLE 2
[0020] Panels (4 in..times.6 in.) of aluminum alloys 2024, 6061,
7075 were coated in accordance with the boric acid-sulfuric acid
anodizing process as described in Boeing Process Specification BAC
5632, "Boric Acid-Sulfuric Acid Anodizing".
[0021] Then a 50 mM yttrium sulfate sealing solution was prepared
by dissolving the yttrium sulfate salt in distilled water and
adjusting to pH 6 using nitric acid at room temperature. The
solution was heated to the boiling temperature which is
approximately 100.degree. C. Panels were immersed in the sealing
solution for 30 minutes.
EXAMPLE 3
[0022] Panels (4 in..times.6 in.) of aluminum alloys 2024, 6061,
7075 were coated in accordance with the boric acid-sulfuric acid
anodizing process as described in Boeing Process Specification BAC
5632, "Boric Acid-Sulfuric Acid Anodizing". Then a 50 mM cerium
sulfate sealing solution was prepared by dissolving the cerium
sulfate salt in distilled water and adjusting to pH 5.5 using
nitric acid at room temperature. The solution was heated to the
boiling temperature which is approximately 100.degree. C. Panels
were immersed in the sealing solution for 15 minutes.
EXAMPLE 4
[0023] Panels (3 in..times.3 in.) of aluminum alloys 2024 and 6061
were coated in 15 wt. pct. sulfuric acid. Then a saturated cerium
acetate sealing solution was prepared by dissolving cerium acetate
salt in distilled water as described by Mansfeld et al., Plating
and Metal Finishing, Dec. 1997, vol. 84 (1997). The solution was
heated to boiling temperature which is approximately 100.degree. C.
Panels were immersed in the sealing solution for 40 minutes. After
sealing, the panels were rinsed with deionized water and air
dried.
EXAMPLE 5
[0024] Panels (3 in..times.3 in.) of aluminum alloy 6061 were
coated in 15 wt. pct. -sulfuric acid. Then a saturated cerium
acetate sealing solution was prepared by dissolving cerium acetate
salt in distilled water as described by Mansfeld et al., Plating
and Metal Finishing, Dec. 1997, vol. 84 (1997). The solution was
heated to approximately 80-85.degree. C. Panels were immersed in
the sealing solution for 40 minutes. After sealing, the panels were
rinsed with deionized water and air dried.
EXAMPLE 6
[0025] Panels (3 in..times.3 in.) of aluminum alloy 7075 were
coated in 15 wt. pct. sulfuric acid. Then a saturated cerium
acetate sealing solution was prepared by dissolving cerium acetate
salt in distilled water as described by Mansfeld et al., Plating
and Metal Finishing, Dec. 1997, vol. 84 (1997). The solution was
heated to approximately 80-85.degree. C. Panels were immersed in
the sealing solution for 20 minutes. After sealing, the panels were
rinsed with deionized water and air dried.
EXAMPLE 7
[0026] Panels (3 in..times.3 in.) of aluminum alloy 2024 were
coated in 15 wt. pct. sulfuric acid. Then a saturated cerium
acetate sealing solution was prepared by dissolving cerium acetate
salt in distilled water as described by Mansfeld et al., Plating
and Metal Finishing, Dec. 1997, vol. 84 (1997). The solution was
heated to approximately 80-85.degree. C. Panels were immersed in
the sealing solution for 20 minutes. After sealing, the panels were
rinsed with deionized water and air dried.
[0027] Rinsing:
[0028] Remove parts from the sealing solution, water immersion
rinse at 50.degree. C. for five minutes, followed by subsequent
rinse at room temperature for five minutes.
[0029] Drying:
[0030] Dry sample with dry oil-free air.
Chemical Concentration, pH, Temperature, And Immersion Time
[0031] The chemical concentration of the dissolved rare earth metal
salt in the sealing solution may be from about 10 mM to about 350
mM. The pH of the sealing solution may be from about 3.0 to about
9.0. The temperature of the sealing solution may be from about
60.degree. C. to the boiling temperature of the sealing solution.
The immersion time in the sealing solution may be from about 10
minutes to about 60 minutes.
Results of Electrochemical Impedance Spectroscopy (EIS) and Optical
Microscopy at 30.times.
[0032] Numerous electrochemical impedance spectroscopy (EIS) runs
were performed to generate Bode plots (logarithm impedance versus
logarithm frequency; phase angle versus logarithm frequency) that
include work on panels of sealed and unsealed coatings made by the
boric acid-sulfuric acid anodizing process. At the end of testing,
the panels were examined at 30.times. magnification to determine
the number of pits and to size the pits as either small or large.
From these data, we selected yttrium sulfate, cerium nitrate, and
cerium sulfate sealing solutions as the more promising candidates.
The selected rare earth metal salt sealing solutions were evaluated
in corrosion and adhesion testing.
Results of Corrosion Testing
[0033] Duplicate 4 in..times.6 in. salt spray panels of alloys
2024, 6061, and 7075 with a coating produced by the boric
acid-sulfuric acid anodizing process were sealed with cerium
nitrate, yttrium sulfate, and cerium sulfate, as in the above
sealing process Examples 1, 2, and 3, respectively. After 336 hours
of salt spray testing, the panels were visually examined. The
passing criterion is that there shall be no more than five pits on
a 3 in..times.10 in. panel or more than nine pits in 90 square
inches of test area. The pit density shall not exceed one pit per
10 sq. in. All alloy 6061 and alloy 7075 panels had one or no pits
on the 24 sq. in. surface. The alloy 2024 panels with yttrium
sulfate and cerium nitrate seal had about five pits per panel,
which is about two pits per 10 sq. in. The alloy 2024 panel with
cerium sulfate had multiple pits.
Results of Paint Adhesion Testing
[0034] Panels of alloys 2024, 6061, and 7075 with a coating
produced by the boric acid-sulfuric acid anodizing process were
sealed with yttrium sulfate, cerium nitrate, and cerium sulfate, as
in the above sealing process Examples 1, 2, and 3, respectively.
Each panel was sprayed with one coat of a paint (manufactured by
Deft) qualified under Boeing Material Specification BMS 10-11,
Grade E, and allowed to cure at room temperature for seven days.
Testing included: dry adhesion, 24 hour wet adhesion, and 7 day wet
adhesion. The passing criterion in the scribe area is that there
shall be no paint lift off {fraction (1/32)} in. beyond the scribe
after the tape adhesion test. The test results showed no paint lift
off from any panel. The three alloys each sealed with the three
different seal solutions all passed the paint adhesion test.
[0035] The patents, specifications, and other publications
referenced above are incorporated herein by reference.
[0036] As will be apparent to those skilled in the art to which the
invention is addressed, the present invention may be embodied in
forms other than those specifically disclosed above, without
departing from the spirit or essential characteristics of the
invention. The particular embodiments of the invention described
above and the particular details of the processes described are
therefore to be considered in all respects as illustrative and not
restrictive. The scope of the present invention is as set forth in
the appended claims rather than being limited to the examples set
forth in the foregoing description. Any and all equivalents are
intended to be embraced by the claims.
* * * * *